1. Field of the Invention
This invention relates to a hand-held radiotelephone for use in a mobile radiotelephone system such as a portable telephone or a cordless telephone system.
2. Description of the Related Art
Among mobile radiotelephone systems available are a cellular-type portable telephone system and a cordless telephone system.
The cellular-type portable telephone system is composed of, for example, a control station CS, a plurality of base stations BS1, BS2 . . . and a plurality of portable units PS1, PS2 . . . as shown in FIG. 12. The control station CS is connected to a wire telephone network NW via a wire communication channel. The base stations BS1, BS2 . . . are connected to the control station CS via wire communication channels CL1, CL2 . . . respectively. The base stations BS1, BS2 . . . form radio zones E1, E2, . . . in different areas, respectively. Each of the portable units PS1, PS2, . . . is connected via a radio channel to the base station in the radio zone to which that unit belongs. This circuit further extends from the base station, passes through the control station CS, and is connected to the wire telephone network NW. In this state, it is possible to communicate with another telephone connected to the wire telephone network NW. When a portable unit has moved into another radio zone during conversation, the control station CS switches the radio channel over which conversation is in progress to the radio channel of the base station in the zone into which the unit has moved, which assures the continuity of the conversation.
On the other hand, the cordless telephone system is such that a base unit corresponding to a base station in the portable telephone system is connected to, for example, a subscriber's line terminal in homes or offices, and that a radiotelephone corresponding to a portable unit is connected to the base unit via a radio channel. This cordless telephone system allows communication in as small an area as within a radius of approximately 100 m of the base unit, outside which communication goes off.
Many radiotelephones used in the portable and the cordless telephone system are of the hand-held type. Since this type of radiotelephone enables the speaker to make a call while holding the radiotelephone itself to his mouth and ear like the handset of a wire telephone, he can use it without a sense of incongruity, feeling it very useful.
Conventional radiotelephones of this type, however, generally have only one operation mode, the handset talking mode as noted earlier. For this reason, for example, when the speaker has his hands full because talking over the phone while taking notes, or when more than one speaker wants to join the conversation, he sometimes has to suffer inconvenience.